Add microceph

For deployments like COS that need S3, it could be handy if concierge was able to install ceph and set up radosgw.
Here's a [reference](https://github.com/sed-i/reusable-justfiles/blob/29c123d0a5b60920c7cf32fd7ef9f49f25e19c25/charm-dev/ubuntu-24.04.just#L156) we're currently using:

```just
microceph:
# Ref: https://canonical-microceph.readthedocs-hosted.com/en/latest/tutorial/get-started/
sudo snap install microceph
sudo snap refresh --hold microceph
sudo microceph cluster bootstrap
sudo microceph disk add loop,4G,3
sudo ceph status

# Ref: https://canonical-microceph.readthedocs-hosted.com/en/latest/reference/commands/enable/#rgw
# (Traefik will take ports 80, 443)
sudo microceph enable rgw --port 8080 --ssl-port 8443
sudo microceph.ceph -s
sudo microceph.radosgw-admin user create --uid=user --display-name=User
sudo microceph.radosgw-admin key create --uid=user --key-type=s3 --access-key=access-key --secret-key=secret-key

sudo apt install -y s3cmd
# Now you can create buckets with:
# s3cmd --host=localhost:8080 --access_key=access-key --secret_key=secret-key --host-bucket= --no-ssl mb s3://loki
```

The issue provides a reusable-justfiles recipe for installing MicroCeph, enabling RADOS Gateway, and creating S3 credentials; start by comparing that reference with concierge's provisioning entry points. No project file or test is named, so done means concierge can perform the requested setup, including the specified RGW ports and S3 access.

Written by the indexing model from the issue text.